1. Field of the Invention
The present invention relates to an apparatus and method for improving flue gas recirculation to minimize the oxides of nitrogen in exhaust emissions.
2. Related Information
Nitrogen oxides (xe2x80x9cNOxxe2x80x9d) are among the primary air pollutants emitted from combustion processes. NOx emissions have been identified to contribute to ground-level ozone formation, visibility degradation, acid rain and human health concerns. As a result environmental regulations have been the main driver forcing industry to install systems to control NOx emissions.
There are two primary sources for NOx generated during combustion: Fuel NOx and Thermal NOx. NOx formed due to conversion of chemically bound nitrogen is referred to as Fuel NOx. Thermal NOx refers to NOx formed from high temperature oxidation (or xe2x80x9cfixationxe2x80x9d) of atmospheric nitrogen. NO is the major constituent of thermal NOx and its formation can be modeled by the Zeldovich equation:
[NO]=k1xc2x7exp (xe2x88x92k2/T)xc2x7[N2]xc2x7[O2]xc2xdxc2x7t
where, [ ]=mole fraction, k""s=constants, T=temperature, and t=residence time. The Oxidation of Nitrogen in Combustion and Explosion, J. Zeldovich, Acta Physiochim, U.S.S.R. (Moscow), 21 (4), pp 577-628 (1946). The equation indicates that NOx formation is an exponential function of temperature and a square root function of oxygen concentration. Thus, by manipulating the temperature or oxygen concentration the formation of thermal NOx can be controlled. The main control strategies for reducing thermal NOx emissions can be characterized into two types: (i) Stoichiometry-based combustion modification systems designed to control the mixing of fuel and air to modify the concentration of oxygen in the flame zone, and (ii) Dilution-based combustion modification systems designed to reduce flame temperature in the flame zone. Post Combustion control of flue gas to remove NOx such as Selective Catalytic Reduction (SCR) and Non-Selective Catalytic Reduction (NSCR) are not only expensive but also operate on a different principle from the present invention.
Stoichiometry-based Combustion Control techniques involve altering the oxygen concentration in the flame zone to lower NOx formation. Examples for stoichiometry-based combustion controls include: Low NOx Burners and Off-Stoichiometric Combustion (e.g., Over Fire Air, and Burners Out of Service). These technologies effectively control NOx emissions by providing air staging to create an initial, fuel-rich zone (partial combustion zone) followed by an air-rich zone to complete the combustion process. Some burner manufacturers also offer fuel staging, which results in ultra low levels of NOx, primarily because they are also designed to recirculate flue gas.
Dilution-based Combustion Control techniques such as Flue Gas Recirculation and Water/Steam Injection control technologies reduce thermal NOx formation by introducing inerts, which absorb heat, thereby, reducing peak flame temperatures. Although dilution methods also reduce oxygen concentration in the flame zone, little reduction in NOx is expected from this mechanism. Water Injection reduces flame temperatures by absorbing the latent heat of vaporization, as such; it results in decreasing the efficiency. Thus, it is mainly recommended as a temporary control measure to reduce NOx during peaking periods.
Flue Gas Recirculation (xe2x80x9cFGRxe2x80x9d) technology, also referred to as Windbox-FGR, does not suffer from this handicap and has minimal impact on efficiency. In a typical Windbox-FGR application, about 10 to 25% of the flue gases are recycled back to the combustion zone resulting in NOx reduction of up to 80%. Recirculating flue gas back to the combustion zone has been one of the most effective methods of reducing NOx emissions from gas and oil fired boilers since the early 1970""s. In conventional applications, the recirculated flue gas is typically extracted from the combustion units outlet duct, upstream of the air heater. The flue gas is then returned through a separate duct and hot gas fan to the combustion air duct that feeds the windbox. The recirculated flue gas is mixed with the combustion air via air foils or other mixing devices in the duct. Windbox-FGR systems require installation of a separate hot gas FGR fan to move flue gas from the boiler exit to the air supply ducting at the windbox inlet, where mixing of the air and flue gas must be uniformly achieved by installation of appropriate mixing devices.
As more stringent rules are applied to reduce NOx emissions by 90%, many existing combustion units, including fired heaters, boilers, ethylene furnaces, incinerators, steam generators, process heaters, and the like, will need to have expensive selective catalytic reduction (SCRxe2x80x94treatment of exhaust gas with ammonia or other reduction or oxidation agents) systems or equivalent post combustion flue gas treatment technologies installed in order to meet the high NOx control levels. The post combustion flue gas technologies such as the SCR system, have an associated pressure drop, so to overcome the pressure drop, a hot gas fan is needed to boost the pressure of the exiting flue gas from levels typically below 1 inch of water to above 3 inches of water. In a typical application, the hot gas fan used by the post combustion flue gas treatment system boosts the pressure of the flue gas so that it can be passed over a catalyst bed to reduce NOx, before exiting through the stack.
The present invention takes advantage of the hot gas fan, used by the flue gas treatment system to reduce NOx, to redirect a portion (slip stream) of the flue gas back into the flame zone. The present invention takes advantage of any type of post combustion fan to redirect a portion of the flue gas back into the flame zone to achieve NOx reduction.
Briefly the present invention comprises an apparatus and process for flue gas recirculation wherein the apparatus comprises: a combustion unit, an exhaust duct for removing flue gas from the combustion unit, a post combustion hot gas fan for exhausting flue gas, and a recirculation line penetrating into the exhaust duct having an extractor for capturing and directing a portion of said exhausting flue gas into said combustion unit for combustion therein. The recirculation line is positioned down stream of said hot gas fan. Preferably the extractor has an opening on a surface facing the hot gas fan to allow the hot gas fan to provide the motive force to the recirculation of the flue gas, i.e., exhaust gas. The hot gas fan may be a forced draft type or an induced draft type. The combustion unit comprises a combustion zone which will preferably comprise a flame burner.
In a preferred embodiment the invention comprises an apparatus and process for flue gas recirculation wherein the apparatus comprises: a combustion unit, an exhaust duct for removing flue gas from the combustion unit, a post combustion treatment unit and a hot gas fan for moving flue gas through said post combustion treatment unit, and a recirculation line penetrating into the exhaust duct between said combustion unit and said post combustion treatment unit, said recirculation line having an extractor for capturing and directing a portion of said flue gas into said combustion treatment unit for combustion therein.
The recirculation line preferably delivers the recirculated flue gas through a diffuser into the intake air going to the burner in the combustion unit, so that a uniform combustion mixture may be obtained.
The term xe2x80x9ccombustion unitxe2x80x9d includes fired heaters, boilers, ethylene furnaces, incinerators, steam generators, process heaters and the like. The term xe2x80x9cductxe2x80x9d as used herein includes ducts, lines, flues, stacks and any equivalent elements.